Marine CSEM in shallow water: Acquisition and interpretation strategies

The marine controlled-source electromagnetic (CSEM) method is being applied to the problem of detecting and characterizing hydrocarbons in a variety of settings. Until recently, its use was confined to deepwater (water depths greater than approximately [Formula: see text]) because of the interaction of signals with the atmosphere in shallower water depths. The purpose of this study was to investigate, using a simple 1D analytical analysis, the physics of CSEM in shallow water. This approach demonstrates that it is difficult to simply decouple signals that have interacted with the earth from those that have interacted with the air using either frequency-domain or time-domain methods. Stepping away from wavelike approaches, which if applied without care can be misleading for the diffusive fields of CSEM, we demonstrate an effective way to mitigate the effect of the air in shallow water surveys by decomposing the EM signal into modes and using only the mode least affected by interaction with the atmosphere. Such decomposition is straightforward in a 1D earth, and we demonstrate that the approach remains valid in higher dimensional structures. We also show that the coupling between signals diffusing through the earth and those that have interacted with air can be used to our advantage in the interpretation of marine CSEM data.

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The marine controlled-source electromagnetic method in shallow water

Geophysics ◽

10.1190/geo2012-0112.1 ◽

2013 ◽

Vol 78 (2) ◽

pp. E67-E77 ◽

Cited By ~ 19

Author(s):

Rune Mittet ◽

Jan Petter Morten

Keyword(s):

Shallow Water ◽

Water Depth ◽

Scattered Field ◽

Approximate Expression ◽

Background Field ◽

Total Field ◽

Controlled Source ◽

Horizontal Electric Dipole ◽

Marine Csem ◽

Reduced Water

We analyzed marine controlled-source electromagnetic (CSEM) acquisition in shallow water by analytic and numerical calculations. The problem at hand was the fact that the amplitude of the airwave due to a horizontal electric dipole becomes larger as the water depth is reduced. The amplitude of the scattered field from a buried thin resistor would be difficult to extract from the increased total field if the scattered-field amplitude were independent of water depth. However, we identified and explained a compensating effect where the amplitude of the scattered field increases with reduced water depth. This amplification effect makes marine CSEM surveys in shallow water feasible. We derived an approximate expression for the total field for a simple but realistic conductivity model. This expression explains why the amplitudes of the term that describes the background field contribution and the term that gives the scattered field from the thin resistor increase when the water depth is reduced. Our results and sensitivity analysis indicated that the sensitivity of marine CSEM data acquired in a water depth of 40 m is comparable to the sensitivity of marine CSEM data acquired in a water depth of 300 m.

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